Developments for complex inelastic large-scale simulations and parameter estimation, with emphasis on large-strain, softening and localization phenomena

This dissertation constitutes a part of an overall research project dealing with the general topic of constitutive modeling of the complex macroscopic behavior of engineering materials, in particular as part of life prediction strategies. Hence, in order to represent realistic material behavior, inelastic deformations and (acting interactively) damage mechanisms are often combined; e.g. using theories of plasticity, viscoelasticity, viscoplasticity, damage/failure mechanics, or other rheological models. Therefore, the need exists for the formulation of a general model for hereditary behavior of inelastic/damaging materials, in the regime of viscous (time/rate dependent) with coupling to damage, under both small and large strain domains.; To this end, the focus here is placed on three important considerations; i.e., (i) theoretical development, (ii) algorithms developments for large-scale simulations, and (iii) material parameters estimation for characterization and practical utilization, of representative class of complex constitutive models, with emphasis on issues pertinent on large strain extensions, softening modeling, and localization phenomena. The main thrust included in this study is developing an efficient and general methodology for computations in the presence of multiplicity of dissipative mechanisms for inelasticity and damage due to strength reductions and stiffness degradations.; More specifically, we utilize two particular classes of models; i.e.; (i) a new multi-mechanism-based model of the coupled viscoelastoplastic-damage type for small-strain applications, and (ii) an extended finite-strain hyperviscoelastic model for large deformation applications. The scope of the work presented is sufficiently general to allow treatment of both isotropic and anisotropic responses (both elastically as well as inelastically), and examples for each will be demonstrated.; From the point of view of mathematical and algorithmic developments, the derivations of the two general classes of models are given, together with the associated stress-integration/material stiffness algorithms that are needed for numerical simulations. The latter are patterned after the forms of UMAT routines (user's defined material subroutines) for applications using the F&barbelow;inite E&barbelow;lement (FE) Code ABAQUS.; On the characterization side, details of the overall strategy for the material parameter estimation are described. This includes the three major phases; i.e., primal response analysis, sensitivity analysis, and optimization. For demonstration purposes, this procedure is applied to the comprehensive characterization (with wide spectrum of stress magnitudes, loading rates, temperatures, and time-scales) utilizing experimental test matrices (with different load, strain, and mixed controls) on a number of different materials, e.g., titanium alloy type (a beta-titanium alloy, TIMETAL 21S), copper type alloy (Cu₄Cr₂Nb), and Nickel-based super-alloy (Waspaloy).; Finally, considering the numerical problems and case studies, results for a rather comprehensive set of truly large-scale simulations are reported. These range from highly-deformable three-dimensional structures undergoing very large strains, to the very complex situations exhibiting localization phenomena due to progressive material softening as a result of stiffness degradation and strength reductions (i.e., post-peak/failure regimes).